Fullscreen HTML5Med Res (??MB)HTML4

SERVICEMAN'S LOG Bits & pieces - odds & ends Yes, it's a bits and pieces month this month. No very profound or heartbreaking technical stories; just a few short ones from which useful lessons were learned and which are worth passing on. The first one did not involve any deep mysteries in the technical sense, but there are a couple of other factors which I feel justify its telling. One of these is the fact that, these days, the problem is relatively rare; the other is that the actual fault is even rarer. As a result, I found myself recalling how I tackled this problem in the early colour days, which led to the idea that a bit of a brush-up in print might be a good idea all round. The set involved was a National TC-1807, a 46cm colour set which belonged to a local motel. And as is often the case with motel installa- tions, there was some suggestion that the fault had been in evidence for some time before it was reported. Guests who stay for only one night seldom report minor problems. Some do not even turn the set on or, if they do, they tend to accept what they get, as long as it talks and moves. Significantly, in this case, the fault was reported by a guest who had booked in for a longer period. And it was, simply, loss of colour; intermittently in the broad sense but often present for hours at a time. In fact, by the time it landed on my bench it had become permanent, a condition for which one is always duly thankful. Mental check list 5E::'RVtCi;;::l\J\/\N AB0UT"f'O t-'c/W(; Pt 'STAB AT l t 44 SILICON CHIP This was when I began recalling my mental check list of possible causes of this condition and the routine for checking them . In one sense, this was easier in early colour sets with discrete components than it is with some modern sets using multi-purpose ICs. But, regardless, there are several fundamental checks which one should strive to make. The first and most obvious is whether there is a video signal, car- rying the chroma information, entering the decoder. Next, is the 4.43MHz crystal controlled reference oscillator functioning? And, at a more subtle level, is it on frequency; ie, within locking range of the burst? This latter fault would be very rare but it has been known to happen. As for the burst itself, this should have already been observed when confirming that there is signal entering the decoder, ideally from a colour bar generator. But the mere presence of the burst is not enough. There is a gating pulse, derived from the horizontal scan frequency, which admits the burst to the crystal phase comparator at the appropriate time. No gating pulse, no burst; no burst, no colour. Another source of trouble is the colour killer. This is designed to block off the chroma circuitry if there is no chroma information, in order to present a monochrome picture free of chroma noise. And, of course, if it kills when it shouldn't, there's no colour. Next there is the half line (7.BkHz) switch, which is used to reverse the reference oscillator phase to the R-Y demodulator, synchronously with the phase reversal introduced at the trans.mitter. Not all receivers respond in the same way to a failure here. In most cases there will still be colour - of a kind - but exhibiting pronounced venetian blind effect. But in some sets it will kill the colour completely. And that's about it. Of course, it is one thing to nominate these sections; it is often a good deal more difficult to pinpoint them, particularly in a modern set with which one is not familiar. In most cases, they will all be contained in a single decoder IC, making access to the actual circuitry impossible. But this is less important than finding and checking the various signals fed to it; eg, the video signal, the crystal reference oscillator signal and , L602 TLK68057 R626 IQK C5i! J., ~R608 0.01 : R625 120 ($ j'330 ~ ,i,, C0-51 C625 50V3.3 C0 -53 VP SYNC f!II C0-:1 "·p :2'1 DbOI DbOZ iCHROMAI ,..-'M-, '-----+-t---iH DblO CL!' _iMAIII 4--------. MA, III.~ ; .. ·w • ~wv-:: ::: c,~1 ),oq •.m. m 5011 -~Albl roo TNP65475AA §)BOARD Fig.1 the relevant portion of the decoder ("B") board, showing IC601 (centre) and the input plug, CO-53, at bottom left. Most of the tests were made around these two units. the burst gating pulse, to name just three. Signal checks So much for the background. With the set on the bench, a perusal of the manual pinpointed the decoder section as being on the "B" board, designated TNP65475AA. This consists mainly of two ICs: IC301 (AN5610N) and IC601 (AN5620N). The latter IC, together with its associated circuitry, performs most of the functions mentioned in the above check list - see Fig.1. I fed a colour bar signal into the set and made my first check at pin 1 of plug CO-53 (lower left), the point of entry for the video signal. All was well here - staircase, sync pulses, burst pulse - and I moved on to the video-in point of IC601 (pin 1). All was well here too, thus clearing the intermediate components, including transformer T601. The next check was at pin 15 of IC601, the entry point for the sync signals (which carry the 10-cycle reference burst), and which come into the board at pin 4 of plug CO-53. (The burst is separated from the sync pulses inside the IC by the burst gate pulse, which is timed to open the gate for the appropriate period). All was correct here, also. OK, what about the burst gate pulse? This comes in on pin 3 of CO-53, and enters IC601 at pin 7, via a rather circuitous route. A quick check with the CRO cleared this. And that, at least at a preliminary level, left only the reference oscillator. This involves pins 12 and 13 of IC601 and consists of the crystal, X601, and capacitors C622, C616 and C617. Again, the CRO is the logical test instrument. Not only will it confirm oscillation but also the amplitude and, with some effort, the frequenc y. A good frequency counter, if available, is a better device for this latter test, assuming a frequency error is suspected. Not that it was necessary in this case. The CRO told the whole story; no oscillation - dead as the proverbial dodo. Looking at the circuit I decided that the crystal was the most likely culprit. And since it needed only a few moments to unsolder it and substitute a spare, this was what I did. And that was it; the set came to life in full colour immediately and hasn't missed a beat since - which was something of an anti-climax I suppose, after all the mental effort that went before. But I don't feel it was wasted; it was a useful refresher course, for me at least. The reluctant Sharp The next story concerns a Sharp 48cm colour TV set, model CX2048. This is a relatively recent model, probably about four years old. The complaint was a little unusual; it was to the effect that if the set was simply switched on and left running, it would run indefinitely - for days, if necessary. (From this I assumed that the owner had done just that). But the catch was that, if it was switched off after a brief period - as little as 15 minutes, as I later established - and then switched on again, it simply would not start. And as I also later established, it needed at JULY1991 45 SERVICEMAN'S LOG - CTD least an hour's rest before it would come good. There seemed little doubt that it was a thermal fault of some kind which involved the starting circuit in the power supply. But I was faced with the problem of not having a manual, or even a circuit. And these things are not always available on demand. However, perusal of the power supply section revealed the presence of an IC with the type number IX0512CE - at least, that's the Sharp designation. Fortunately, I r.13cognised it by what might be termed its generic type number: STR41090, an IC which, among other things, features a starter mechanism. Which brings me to the first point of this story. Sharp - and some other makers - make a habit of this; taking a standard component and changing its generic type number to one of their own. I won't dwell on the philosophy behind this but it can mean that the generic version costs less than the special version. There can be other differences too, which I will expand on later. There have been a number of cross references published, listing these special/generic combinations, many of wpich I cannot recall off-hand. But one I do recall was by TESA (Television & Electronic Services Association Pty Ltd, 245 Rainbow Crescent, Coogee, NSW) and, in particular, the issue for October 1989. Anyway, having established that much, my attention was drawn to two components: a lµF 100V electrolytic capacitor and a 4.7kQ 0.5W resistor in series. This RC circuit was connected to pin 5, which I knew to be the pin for the starter circuit. My suspicion, while somewhat speculative, was further supported by the general suspicion most of us entertain concerning low-value electros. As it transpired, my suspicion was fully justified - well, sort of. So I changed them, even though superficial tests revealed nothing wrong. The trouble was, it didn't work. So I was back to square one and, without at least a circuit, in a pretty hopeless situation. Help sought At this point, it occurred to me to contact a colleague in a distant sub- urb, who is something of a Sharp specialist. Perhaps he knew the answer but, in any case, I could put the bite on him for a copy of the circuit. So I rang him and outlined the problem. His answer was immediate. "The one mike electro and the four point seven 'k' resistor to pin 5 of IC701 change them". "I already have. It didn't do any good", I replied. There was silence at the other end, so I went on, "Could it be a faulty IC?" "It could be, I suppose, but I've never had one. It's always one of those two components - usually the electro. Not surprising really, considering how close it is to the heatsink". And that raised another point which I will deal with later. By this stage, there wasn't much more my colleague could suggest but he did offer to send me a copy the circuit. Well, that was something. And he was as good as his word; the circuit turned up in the next mail. Not that it helped much really, except to clarify the arrangement in my own mind. The two components I had replaced were C711 and R708 and, as my colleague had already suggested, the IC was IC701. I went over the circuit seeking fresh inspiration but found none. So, after a few routine checks I plumped for the :,, WOUL.P YOIJ, UH, L-IKG: ,--o s~e MY He:-Ai"'SIN ' W/;Ll.. AWA OM 46 SILICON CHIP TETIA TV TIP AWA AV-11 Thorn TR101 video cassette recorders. Symptom: The most obvious symptom was that the clock would not run. It could be set, and the counter would work, but the clock would not advance. A secondary fault was that the set would sometimes not switch on. Cure: IC 8AO, an M558858 micro processor that controls the timer was faulty. The uPC runs the clock and counter, and also controls the power on and off for use with the timer record mode. (There are two ·'M558858 uPC's in this model. The timer one carries the suffix -615P). TETIA TV Tip is supplied by the Tasmanian branch of the Electronic Technician's Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, 7015. IC. I put one on order (the STR version), it arrived in a few days, and I fitted it. Over the next few days I put the set through as many variations of the onoff cycle that I could think of and it never once failed to start. So it was . returned to the customer and, at last check, is still performing faultlessly. But the second point to be raised about this set - prompted by my colleague's remark - is really one of design. Why is that lµF electro mounted in such close proximity to the heatsink carrying the IC107? These small value electros are notorious for their tendency to dry out prematurely, so why aggravate this by turning on the heat? Nor is this a one-off situation, as I will demonstrate later. Another Sharp In fact , this story reminded me of the next one, which happened some time ago, and which was similar in many ways. It was another Sharp, a 63cm up-market type with stereo sound, type DV-6384. The main symptom was similar; intermittent failure to start but, in this case, apparently not temperature dependent. The circuit is quite different, using an IC designated by Sharp as IX0308CE but also available under the generic type number, STK7310. (There's another story about that too, which I will detail later). Perhaps coincidentally, it is also designated as IC701. In this case there are several electrolytic capacitors associated with the IC and, as before, they are mounted in close proximity to the IC heatsink. And, acting on the advice ofmy aforementioned colleague, I simply changed them all. That seemed to fix it, at least to the extent that I could test it in the workshop . So it went back to the customer and all was well for several weeks. Then it was back with the report that it was still faulty "occasionally". This time I plumped for the IC more in desperation than anything and that did fix it. I still have a suspicion that there were two faults; a faulty electro and a faulty IC. Be that as it may, I still feel that the placement of those electros represents poor design. And that thought takes me back still further. This same set, a couple of years earlier, suffered from a form of foldover, whereby all the text and other rubbish in the horizontal blanking period was running across the centre of the screen. I made a couple of tentative stabs at it but to no avail. I spoke to the Sharp service department twice about it, but none of their suggestions helped. So rather than waste more time, I turned to my Sharp specialist colleague on that occasion also. "Ah", he chuckled, "that's easy" (yeah, easy when you know how). "It's in the vertical output stage; transistors Q501 and 502. Change all the electros around these". And he was right. I changed them and that fixed it. But the real point about the problem, as he pointed out at the time, is the mounting of the electros. Each of these two transistors is mounted on the bottom of a fairly large U-shaped heatsink, the two heatsinks being mounted side by side, with about 25mm between the upright sections. This is fair enough in itself, but there are also some six or seven electros mounted in the vicinity; some actually inside the heatsinks and several in the 25mm space between them. It's little wonder they pack up. In fact, my colleague suggested that I follow his practice and relocate the electros under the board, well away Australian Made TEST EQUIPMENT YOU CAN TRUST SHORTED TURNS TESTER Built-in Meter to check EHT transformers incl uding split diode type, yokes and drive transformers. $78.00 + $3.00 p&p DEGAUSSING WAND Strong magnetic field, larger than usual coil with multicore centre. Double insulated for your safety, also fitted with momentary on/off switch. 240V A/C 2.2 amps. Just about as important as having a soldering iron in your toolbox! $75.00 + $10.00 p&p HI-VOLTAGE PROBE Built-in meter reads positive or negative 0-50kV. For checking EHT and focus as well as any other Hi-tension voltages. $98.00 + $5.00 p&p REMOTE CONTROL TESTER (INFRARED OR ULTRASONIC) Designed to test lnfrared or Ultrasonic control units. With the extension lead you can also test infrared units which cannot be placed in front of the testing unit. Requires a 9V battery. Output is via the LED diode and piezo speaker. $85.00 + $4.00 p&p LASER DETECTOR PROBE A new addition to the remote control tester. Comparable with units costing $500 or more. You can test the laser pick-up in compact disc players. $27.00 + $2.00 p&p NEW PRODUCT TEST TUNER Save time and money with this tester. Helps you determine if you have a tuner problem, an AGC problem or a fault in the IF stages. $250.00 (Tax inc.) TUNER REPAIRS from $17.00 exchange + p&p Cheque, Money Order, Bankcard or Mastercard -a;v. TUNERS} .·.·-•-•.•.·.·.·.·.--·-·-·-·-·-·.-.·.·.·.·-·.·.·.·.·.·.•.•.·.•.•-·.·.•.•-·,:'.:-:-:-· 216 Canterbury Road, Revesby, NSW, AUSTRALIA, 2212 ~ (02) 774 1154 '~ (02) 774 1154 JULY 1991 47 SERVICEMAN'S LOG - CTD from the heatsink. I followed his ad vice and there has been no sign of that problem since. Part numbers Now, let's go back to the matter of special/generic type numbers and, particularly, the IX0 308CE/STK7310 combination. The STK7310 is in fact an upgraded version of the original IX0 308CE w hich had a nasty habit of developing an internal short circuit. As I can personally testify, the resu lts could be catastrophic. I have encountered two DV-6384 sets (one is still in my workshop) which were total write-offs after such fai lures. Apparently, protective circuits in these sets couldn't act fas t enough and the breakdown took out ICs and transistors left, right and centre. The estimated cost of repairs were such that the owners couldn 't consider it. From time to time I have considered trying to salvage the set in the workshop, on a purely personal basis. But when I think of the many hours of work involved and the cost of the replacement parts, I tend to back off On the other hand , it could make a good source of spare parts - those that have survived, that is. In fairness, I should add that these two sets were early production models and I have no doubt that later models used the upgraded IC , regardless of the type number it carried. Believe it or not To round off, here is the you wouldn 't-believe-i t story of the month. From time to time, I have chronicled the weird assortment of foreign objects that I have found in video recorders; coins, match sticks , parts of children's toys, dead spiders - to name just some. But I reckon this one beats them all. Readers may recall that I have a friend who, as well as keeping his own assortment of TV sets in running order, sometimes helps out a friend or neighbour on a purely non-professional basis (s ee April 1990). Such a situation arose a few months ago when a fri end asked him to have a ~.RADIO PTY. LTD .la1S~.r.~1;r.:.i;: RCS Radio Pty Ltd is the only company which manufacture s and sells every PCB f., front panel published in SILICON CH IP, ETI and EA . 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. 48 SILICON CHIP look at his colour TV set. It was a German make , a Gratez , which ap peared briefly on the Australian market with the introduction of colour. Since he had another set, he was happy for my friend to work on it on a "when y'got time basis". Which was just as well, because it needed a fair amount of work plus some spare parts which had to be tracked down. Anyway, the job was eventually completed and the set returned to the friend and set up in the lounge room. It turned in an excellent performance and everyone was delighted. Then came anoth er request. The owner had a video recorder which was normally used with the set but which had been put away in a cupboard while the set was being serviced . Would my friend supervise the various cable connections necessary to get the recorder back into operation? "Of course, no problem". But there was a problem. When all the necessary connections had been completed and an attempt made to play a tape, it simply wouldn 't work. More precisely, it wouldn't accept the video cassette. So my friend grabbed the appropriate screwdriver and whipped the cover off. And what did he find? During the storage period a colony of ants had found their way into the cupboard and then into the recorder. And they had built a substantial nest in the mechanism, consisting mainly of eggs and, of course, the ants themselves crawling all over them. After the initial shock came the important question as to how best to tackle the problem. A brute force approach did not seem to be appropriate, since there was a risk of adding to whatever damage might have been done already. So my friend simply took the recorder outside and sat it in the Sun for an hour or so. And that was the best idea. The ants didn't like the change of venue and proceeded to methodically remove themselves and the eggs as only they knew best. Finally, when the site had been abantloned, my friend carefu lly cleaned out what little rubbish remained, examined the mechanism for any signs of damage, found none, and tried the recorder once more. And it worked perfectly, first time; which was nearly as big a surprise as finding the intruders in the first place. But at least it was a happy ending.SC